1. Field of the Invention
The present invention relates to a relatively small-volume fluid delivery pump wherein two plungers reciprocate to deliver a fluid; i.e, a so-called small-volume plunger reciprocating type fluid delivery pump which may be used, for example, to deliver a mobile phase in liquid chromatography.
2. 2. Description of the Related Art
A typical conventional small-volume plunger reciprocating type fluid delivery pump has a driving motor, plungers for driving two pump heads, respectively, and a converting mechanism for converting the rotational motion of the driving motor into a reciprocating motion of each plunger.
FIG. 4 shows the plunger speed characteristics with respect to an angle .theta. of a conventional small-volume plunger reciprocating type fluid delivery pump having two pump heads.
The curve 1 shows the plunger speed characteristics of the first pump head, while the curve 2 shows the plunger speed characteristics of the second pump head. A cam is used as a converting mechanism for converting the rotational motion of the driving motor into a reciprocating motion of each plunger. The axis of ordinates, i.e., dr/d.theta. (r is the distance from the center of rotation of the cam), represents the plunger speed at the time when the driving motor is rotating at a constant velocity. The upper and lower sides of the axis of abscissas show the plunger speeds at the deliver and suction sides, respectively.
The cam is shaped such that dr/d.theta. shows a trapezoidal pattern.
In the case of an ideal fluid whose delivery flow rate is proportional to the delivery-side plunger speed, the flow rate of the fluid being delivered, which is the sum of the respective delivery flow rates of the two pump heads, is constant throughout all angles of the cam when the driving motor is rotated at constant velocity, as shown by the reference numeral 3 in FIG. 4.
In practice, however, it is impossible to deliver a fluid at the beginning of the trapezoidal delivery cycle if the pressure is excessively high due to the compressibility of the fluid, the delay in response of a check valve and other factors, and deficiencies 4 and 5 occur in the delivery flow rate and the pressure as shown in FIG. 5. The result is a periodic pulsing flow which leads to a noise in a detection which must be carried out with a high sensitivity.
In order to reduce pulsations in the delivery of fluids, it is conventional practice to control the speed of rotation of the driving motor such that, during the beginning of delivery where the pressure may be insufficient the motor is rotated faster than in the other periods of the cycle.
More specifically, U.S. Pat. No. Re. 31,608, Magnusse Jr., discloses a fluid pump mechanism for delivering fluid against a back pressure which comprises a piston movable within a chamber for drawing fluid into the chamber during a chamber filling interval, pressurizing the fluid during a pressurizing interval wherein the fluid pressure attains an effective delivery value prior to delivery from the chamber and delivering the pressurized fluid from the chamber during a delivery interval of piston movement, and means for controlling the rate of piston movement such that the piston moves at a predetermined rate during delivery of the pressurized fluid. The controlling means includes a "pump-up" means for establishing a greater rate of piston movement during the pressurizing of the fluid, for signaling completion of fluid pressurization, and for thereupon establishing the predetermined rate of piston movement for effecting delivery of the pressurized fluid from the chamber, thereby increasing the time during which fluid is delivered to a receiving system and decreasing the time of filling or refilling and pump-up prior to such delivery, and thus enabling fluid to be delivered at a given flow rate and with a greatly reduced pulsation.
U.S. Pat. No. 4,045,343, Achener et al., discloses a high pressure liquid chromatography system including a reservoir for a liquid mobile phase, an LC column and detector, a high pressure reciprocating pump for enabling flow from the reservoir through the column, and a positively actuated inlet valve for controlling flow from the reservoir to the pump chamber. The pump is driven by motor means, such as a stepping motor, directly coupled thereto; and the inlet valve is actuated by the power train of the motor and pump, e.g., by an eccentric carried by the pump crank shaft. The pump piston is similarly driven by an eccentric, the pump and inlet valve eccentrics being angularly displaced in their respective positions at the crank shaft, as to delay opening of the inlet valve for a predetermined period following a pump stroke, in order to enable decompression of the liquid in the pump chamber. The average rotational velocity of the stepping motor is controlled throughout each full crank shaft rotation, so as to enable a precisely selected cycle of pump operation. In particular, the speed of the motor is so regulated in conjunction with the mechanical actuation of the pump piston and inlet valve as to provide (at the low flow rates where such behavior is critical) a very short duration fill period--which implies a rapid withdrawal of the piston or plunger from the pump cylinder. Thereafter, the second portion of the pumping cycle, which corresponds to pumping or displacing the liquid from the pump toward the chromatographic column, is effected under crank shaft rotation (as a function of time) such that the axial displacement of the piston is relatively linear as a function of time.
FIG. 6(A) shows compensation for pulsations in the case where a fluid is delivered under high pressure, while FIG. 6(B) shows pulsation compensation in the case where the delivery of a fluid is effected under low pressure. The reference symbol a represents a pulsing flow in the case where the speed of rotation of the cam is kept constant; b represents the speed of rotation of the cam controlled so as to compensate the pulsing flow; and c represents the pulsing flow thus compensated.
The higher the pressure, the greater the deficiency in pressure; therefore, as the pressure is increased, the rotational speed of the driving motor must be increased correspondingly.
Thus, the prior art method wherein the rotational speed of the driving motor is varied to reduce pulsations suffers from the problem that, as the pressure is increased, the rotational speed of the driving motor must be increased correspondingly to make compensation, and therefore the load on the motor increases and a driving motor having a relatively high output is needed.
A piston pump has a pump chamber having an inlet check valve and an outlet check valve in order to allow liquid to be pressurized to flow in one direction. A piston reciprocally moves within a pump chamber so that the liquid can be made to flow. A pump chamber seal partially surrounds the piston in order to prevent the liquid from leaking from the pump chamber. This means that the piston reciprocates with the rubbing of the pump chamber seal constantly. Although a pump chamber seal is provided, leaking by a small amount of liquid from the seal cannot be avoided due to the above-described mechanism.
In a liquid chromatograph, sometimes salt solution, such as, potassium phosphate water solution is conducted by the pump. When salt solution leaks from the pump chamber seal, the salt crystallizes on the backside of the seal over a long period of time, and, the crystallized salt tends to scratch the seal and detrimentally affects the pump's performance.
A pump which has a rinse chamber adjacent to a pump chamber is known. Water is supplied through the rinse chamber, thereby preventing salt crystallization on the seal. FIG. 11 shows such a pump which is provided with the rinse chamber.
Referring to FIG. 11, a piston 102 reciprocates within pump chamber 104. A pump chamber seal 106 partially surrounds the piston 102. A rinse chamber 110 is provided and includes a pump chamber seal 106 and a rinse chamber seal 108. The rinse chamber 110 has an inlet conduit and an outlet conduit. A syringe is connected to the inlet conduit. Water for rinsing is supplied by the syringe 112. Also, a chromatographic pump equipped with another pump for the rinse solution, instead of the syringe as shown in FIG. 11, is known.
However, a liquid chromatographic pump which has a rinse chamber equipped with a syringe for rinse water cannot regularly rinse a piston and a seal. Similarly, the liquid chromatographic pump which has a rinse chamber equipped with another pump for rinse water is fairly expensive.